Background Collaterals to occluded infarct-related coronary arteries (IRA) have been observed after the onset of acute ST-elevation myocardial infarction (STEMI).We sought to investigate the impact of early coronary collateralization,as evidenced by angiography,on myocardial reperfusion and outcomes after primary percutaneous coronary intervention (PCI).Methods Acute procedural results,ST-segment resolution (STR),enzymatic infarct size,echocardiographic left ventricular function,and major adverse cardiac events (MACE) at 6-month follow-up were assessed in 389 patients with STEMI undergoing primary PCI for occluded IRA (TIMI flow grade 0 or 1) within 12 hours of symptom-onset.Angiographic coronary collateralization to the occluded IRA at first contrast injection was graded according to the Rentrop scoring system.Results Low (Rentrop score of 0 or 1) and high (Rentrop score of 2 or 3) coronary collateralization was detected in 329 and 60 patients,respectively.Patients with high collateralization more commonly had prior stable angina and right coronary artery occlusion,but less often had left anterior descending artery occlusion.At baseline,these patients presented with less extent of ST-segment elevation and lower serum levels of creatine kinase myocardial band (CK-MB) and cardiac troponin Ⅰ (cTnl).Procedural success rate,STR,corrected TIMI flame count,and area under the curve of CK-MB and cTnl measurements after the procedure were similar between patients with high collateralization and those with low collateralization (for all comparisons P＞0.05).There were no differences in left ventricular ejection fraction and rates of MACE at 6 months according to baseline angiographic collaterals to occluded IRA.Conclusions In patients with acute STEMI undergoing primary PCI within 12 hours of symptom-onset,coronary collateralization to the occluded IRA was influenced by clinical and angiographic features.Early recruitment of collaterals limits infarct size at baseline,but has no significant impact on myocardial reperfusion after the procedure and subsequent left ventricular function and clinical outcomes.